Coreceptor shift occurs in approximately half of all HIV infected individuals, and is promoted by use of CCR5 antagonist therapy. The proposed work uses a unique panel of CCR5/CXCR4 chimeric coreceptors and a panel of coreceptor shift HIV-1 isolates to examine the critical domains of CXCR4 required for HIV entry. The methodology includes ultra dense sequence analysis of HIV-1 gp120 sequences of quasispecies from U87.CD4 cells infected with env amplicons from a panel of dual/mixed isolates selected during CCR5 antagonist therapy in patient-specific virus libraries. Aim 1. What proportion of HIV-1 gp120 sequences are responsible for VCVr resistance and how do HIV- 1 gp120 sequences of VCVs and VCVr isolates differ? For this purpose, U87.CD4.R5.X4 cells will be infected with patient-specific virus libraries in the presence or absence of VCV, and infected cell DNA subjected to ultra dense sequence analysis. Aim 2. What proportion of HIV-1 gp120 sequences utilize CXCR4 for entry and how to the gp120 sequences differ from those that can not utilize CXCR4? For this purpose, U87.CD4.R5 and U87.CD4.X4 cells will be infected with patient-specific virus libraries, and infected cell DNA subjected to ultra dense sequence analysis. Aim 3. What are the minimal domains of CXCR4 utilized for infection and how do HIV-1 gp120 sequences utilizing ECL3 or ECL2 differ from those that do not utilize these domains? For this purpose, U87.CD4 cells expressing chimeric CCR5/CXCR4 coreceptors will be infected with patient-specific virus libraries, and infected cell DNA subjected to ultra dense sequence analysis. Molecular details on the use of minimal domains of CXCR4 required for HIV entry will provide critical information to guide the development of more effective antiviral agents.